Network devices such as switches, routers, bridges and suchlike are commonly organised so that they can conform to a variety of different transmission protocols appropriate for the transmission of data packets over different transmission media at a selected data rate or a variety of data rates Where a device is intended to be capable of transmission or reception of data over a fibre optic link it is normal practice for the receiving channel of the optical module which provides for connection to a fiber optic link to include a signal detection circuit which provides a ‘signal detect’ output when the receiving channel is receiving signals in optical form over the link between the device and a remote device at the other end of the transmission link Such a ‘signal detect’ signal must be coupled to a dedicated pin, typically a terminal pin on the application specific integrated circuit which constitutes most or all of the operating circuits of the network device Very typically, each port of such a device may require transmission and reception pins for coupling by way of an appropriate connector to the fiber cable, further pins for connection to the transmit and receive channels of the optics module and the additional signal detect pin Devices which have a large number of ports and therefore terminal pins have a substantial number of signal detect pins.
A fibre optic module employed for connection between a port of a network device and a fibre optic medium commonly employs an automatic gain controlled amplifier in the receiving channel of the optical module The receiving channel is inevitably in close proximity to the transmitting channel and accordingly noise from the transmission side may be detected by the receive side, amplified by the automatic gain control amplifier and interpreted as genuine data from the link As is explained later, such a link may potentially auto-negotiate with itself and/or otherwise interpret that the link status is ‘true’ when auto-negotiation is disabled To prevent this from happening. and to determine when the fibre optic link is in a proper state for transmission of signals, a signal called ‘signal detect’ is employed for each port of the device that is coupled to the optics module This signal is fed back to the ASIC and used to indicate that the link is ‘up’ (properly operative) for the port.
One objective of the present invention is to avoid a false indication of satisfactory status arising from detection by the receive side of an optics module of signals in fact sent from the transmitting side or channel of the same module. A further object of the invention is to reduce the number of pins per port in a practical system, especially where the ASICs become larger and more complex.
Auto-negotiation is the conventional term used for a technique according to which devices at each end of a link ‘advertise’ their technical abilities. such as the rate or rates at which they can operate, and to perform an arbitration which will establish the common modes that exist between the two devices and to allow the devices to resolve to a single mode of operation using a predetermined priority resolution function The basic mechanism which is used to achieve auto-negotiation is to pass information encapsulated within a burst of closely spaced link integrity test pulses, commonly called fast link pulses when auto-negotiation is performed according to IEEE Standard 802 3 Broadly, since it is not intended necessarily to limit the present invention to operation according to that Standard, auto-negotiation requires various state machines or their equivalent which will generate the relevant link pulses and have recourse to various registers that contain coded signals identifying the technological abilities of the relevant device and further registers which will contain the technological abilities or possible modes of operation of the device at the other end of the link, as determined by an examination of received link pulses The auto-negotiation function has recourse to these registers in performing the required arbitration As indicated, auto-negotiation is a well established function the subject of detailed description and definition Reference should be made particularly to chapter 28 of IEEE Standard 802 3 (December 1998) for detailed information on the manner in which auto-negotiation can be conducted.
Included in the known features of auto-negotiation is the transmission of ‘Pages’, namely coded signals within the aforementioned link pulses These pages include a ‘Base Page’ of which the prescribed fields indicate the various technological abilities of the relevant device Auto-negotiation also includes a provision for ‘Next Pages’ A Next Page function can be used within auto-negotiation arbitration to allow exchange of arbitrary pieces of data The exchange of Next Pages occurs after the base link code words have been exchanged and transmission ends when both ends of a link set their ‘Next Page’ bits to logic 0 to indicate that neither end has anything additional to transmit Again. reference may be made to chapter 28 of the aforementioned Standard for a specific example of auto-negotiation employing Next Page transmission.